remote sensing

Article Dynamics of Dalk in East Derived from Multisource Satellite Observations Since 2000

Yiming Chen 1,2 , Chunxia Zhou 1,2,*, Songtao Ai 1,2 , Qi Liang 1,2, Lei Zheng 1,2, Ruixi Liu 1,2 and Haobo Lei 1,2

1 Chinese Center of Surveying and Mapping, Wuhan University, Wuhan 430079, China; [email protected] (Y.C.); [email protected] (S.A.); [email protected] (Q.L.); [email protected] (L.Z.); [email protected] (R.L.); [email protected] (H.L.) 2 Key Laboratory of Polar Surveying and Mapping, Ministry of Natural Resources of the People’s Republic of China, Wuhan 430079, China * Correspondence: [email protected]

 Received: 26 April 2020; Accepted: 1 June 2020; Published: 3 June 2020 

Abstract: Monitoring variability in outlet can improve the understanding of feedbacks associated with calving, ocean thermal forcing, and climate change. In this study, we present a remote-sensing investigation of Dalk Glacier in East Antarctica to analyze its dynamic changes. Terminus positions and surface ice velocities were estimated from Landsat and Sentinel-1 data, and the high-precision Worldview digital elevation model (DEM) was generated to determine the location of the potential ice rumple. We detected the cyclic behavior of changes and similar periodic increases in surface velocity since 2000. The terminus retreated in 2006, 2009, 2010, and 2016 and advanced in other years. The surface velocity of Dalk Glacier has a 5-year cycle with interannual speed-ups in 2007, 2012, and 2017. Our observations show the relationship between velocity changes and terminus variations, as well as the driving role of the ice rumple. Ice velocity often increases after calving events and continuous retreats. The loss of buttressing provided by an ice rumple may be a primary factor for increases in ice velocity. Given the restriction of the ice rumple, the surface velocity remains relatively stable when the glacier advances. The calving events may be linked to the unstable terminus caused by the ice rumple.

Keywords: glacier dynamic; glacier velocity; terminus position; ice rumple; multisource satellite data; Dalk Glacier; Antarctica

1. Introduction The floating ice shelves, which cover over three-quarters of the periphery of Antarctica, play important mechanical roles in buttressing the outlet glaciers of the [1,2]. These ice shelves are highly sensitive to a changing climate due to direct contact with the ocean [3–5]. Calving events or reductions in thickness of ice shelves have been observed to accelerate upstream tributary glaciers dramatically [6,7], contributing to mean global [8,9]. In recent years, many floating ice shelves and outlet glaciers have changed rapidly. Flow acceleration and ice thinning of vast ice shelves have been observed in West Antarctica, much of which is a response to oceanic forcing [10–12]. On the Antarctic Peninsula, major ice shelves have collapsed catastrophically and retreated significantly due to warming oceans, rising atmospheric temperatures, and declining sea ice, thereby resulting in the acceleration and thinning of upstream glaciers [6,13–16]. In East Antarctica, thinning has also been reported in some large ice shelves [10,12,17], and the dynamic changes of several outlet glaciers were confirmed to be related to sea ice conditions, subglacial floods, or intense melting [18–21]. During the period of 2000–2012, outlet glaciers in displayed

Remote Sens. 2020, 12, 1809; doi:10.3390/rs12111809 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 1809 2 of 18 an anomalous signal of ice front retreat [18], which is linked to sea ice changes. The acceleration of in 2009 was observed during a lake drainage event [19]. The ice velocity of the Polar Remote Sens. 2020, 12, x FOR PEER REVIEW 2 of 17 Record Glacier, which is sensitive to melt at both the surface and base, increased significantly during 2005–2015stable state [21 ].with Most irregular outlet temporal glaciers in variations East Antarctica (e.g., Totten are in Glacier) a relatively [22] or stable periodic state rebirth with irregular of ice temporaltongues variations (e.g., Mertz (e.g., Glacier) Totten [23]. Glacier) [22] or periodic rebirth of ice tongues (e.g., ) [23]. StudyingStudying changes changes in outletin outlet glacier glacier dynamics dynamics and flowand regimesflow regimes of ice shelves of ice canshelves help understand,can help quantify,understand, and evaluate quantify, their and constrainingevaluate their e ffconstrainingects on upstream effects iceon flows.upstream Ice shelvesice flows. in Ice Antarctica shelves in lose massAntarctica mainly fromlose themass calving mainly of icebergs from the and calving basal melt, of andicebergs both areand of approximatelybasal melt, and equal both importance are of acrossapproximately the whole equal ice sheet importance [3,12]. Thinningacross the andwhole calving ice sheet events [3,12]. have Thinning been widely and calving investigated events have along thebeen margins widely of Wilkesinvestigated Land [along11,18]. the In addition,margins of regional Wilkes outletLand glaciers[11,18]. mayIn addition, be susceptible regional to outlet coupled ocean-climateglaciers may forcing be susceptible and various to coupled local obstacles,ocean-climat suche forcing as ice risesand various or ice rumples local obstacles, [24,25]. such Ice rumplesas ice orrises pinning or ice points rumples are small-scale[24,25]. Ice rumples grounded or pinning features, points which are significantly small-scale impactgrounded the features, grounding-zone which dynamicssignificantly and impact stability the of grounding-zone ice shelves [26– dynamics28]. They and provide stability substantial of ice shelves sources [26-28]. of buttressing They provide to ice shelves,substantial but the sources process of andbuttressing evolution to remain ice shelve poorlys, but understood the process [24 ,and29]. evolution remain poorly understoodTo better understand[24,29]. the mechanisms of change in regional outlet glaciers, we use high temporal and spatialTo better resolution understand multisource the mechanisms satellite data of change to derive in regional 20-year outlet long glaciers, time series we ofuse terminus high temporal changes andand velocity spatial variationsresolution multisource for Dalk Glacier satellite in data East to Antarctica. derive 20-year Combining long time theseries high-resolution of terminus changes surface and velocity variations for Dalk Glacier in East Antarctica. Combining the high-resolution surface elevation from Worldview-1, we determine the location of the ice rumple that buttresses upstream ice elevation from Worldview-1, we determine the location of the ice rumple that buttresses upstream flow and affects glacier terminus stability. ice flow and affects glacier terminus stability. 2. Study Site 2. Study Site Dalk Glacier (69 25 S, 76 27 E), or Dålk Glacier, is a 15 km-long outlet glacier located in the Ingrid Dalk Glacier (69°25◦ 0 ′S, 76°27◦ 0′E), or Dålk Glacier, is a 15 km-long outlet glacier located in the , East Antarctica (Figure1). It drains into the southeast part of Prydz Bay between Christensen Coast, East Antarctica (Figure 1). It drains into the southeast part of Prydz Bay between the and Steinnes, forming a floating that is approximately 8 km long and the Larsemann Hills and Steinnes, forming a floating ice tongue that is approximately 8 km long and 3 km3 km wide. wide.

Figure 1. (a) and (b) show the location of Dalk Glacier along the Ingrid Christensen Coast, East Figure 1. (a) and (b) show the location of Dalk Glacier along the Ingrid Christensen Coast, East Antarctica. (c) An overview of Dalk Glacier. The background is a Worldview-2 image acquired on 2 January 2016. Blue lines indicate the location of the grounding line. Black dots are ICESat/GLAS

Remote Sens. 2020, 12, 1809 3 of 18

Antarctica. (c) An overview of Dalk Glacier. The background is a Worldview-2 image acquired on 2 January 2016. Blue lines indicate the location of the grounding line. Black dots are ICESat/GLAS altimeter data from 19 March 2008. Red triangle points are radio-echo sounding data acquired in 1990/91.

Previous studies have focused on the morphology and ice velocity of Dalk Glacier [30–32], but research on the long-term dynamic changes and driving forces is limited. Landsat and Radarsat images were used to monitor the terminus locations in 1973–1997, and a frontal disintegration was discovered 1 in 1988. The average annual velocity for 1990–1997 was 190.55 m a− at the front of Dalk Glacier [30]. During the 21st and 24th Chinese National Antarctic Research Expeditions, several monitoring marks were placed on the surface of the glacier by helicopters. Surface velocities from 2007 to 2012 were evaluated based on the forward intersection method, and no obvious yearly ice-flow increases were found [32]. During this interval, seasonal velocity changes and ice-flow acceleration were observed in 2009 when an ice-calving event occurred [33]. Dalk Glacier showed alternating patterns between advance and retreat from 2000 to 2016 [31].

3. Data We employed multisource satellite data to study the changes in terminus positions and surface ice velocities of Dalk Glacier in 2000–2019 (Table1). Images acquired by the Enhanced Thematic Mapper Plus (ETM+) mounted on Landsat 7 and the Operational Land Imager mounted on Landsat 8 were used to investigate the annual ice velocity of Dalk Glacier. All Landsat images were obtained during the austral summer and with cloud cover under 10%. The monthly surface velocities were measured from 20 scenes of Sentinel-1 synthetic aperture radar (SAR) images acquired from August 2016 to September 2017. The Reference Elevation Model of Antarctica (REMA) dataset [34] was used as the reference DEM for geocoding and coregistering the Sentinel-1 imagery employed in the intensity offset tracking technique. The freely available Global Land Ice Velocity Extraction from Landsat (GoLIVE) dataset [35] from October 2015 to February 2017 was used as a supplement to the calculated surface velocities. All cloud-free Landsat images acquired between 2000 and 2019, and Sentinel-1 images acquired between August 2016 and September 2017 were employed to map glacier margins and extract terminus positions.

Table 1. Multiple sensors for glacier terminus and surface velocity.

Tracking Patch Platform Sensor Band Step (pixels) Time (pixels) Landsat 7 ETM+ Pan 256 to 64 8 2000–2012 Landsat 8 OLI Pan 256 to 64 8 2013–2019 Sentinel-1A/B TOPS IW C band 640 128 100 20 201608–201709 × ×

We used the 0.5 m-resolution stereo panchromatic dataset acquired by Worldview-1 on 6 October 2015 to generate surface elevations of Dalk Glacier. The altimeter data (GLA12 L2 Global Altimetry Data Version 34) [36] from the NASA ICESat GLAS instrument, acquired in 2004–2008, and the REMA dataset were used to validate the Worldview DEM (WVDEM). To evaluate the floating condition of the ice tongue, we used airborne radio-echo sounding (RES) measurements acquired in 1990/91 from the Australian Antarctic Data Centre (AADC) [37] to analyze the difference between the actual ice thickness and the equivalent ice thickness calculated from surface elevation. Remote Sens. 2020, 12, 1809 4 of 18

4. Methods

4.1. Terminus Position The terminus positions of Dalk Glacier were delineated, and changes were estimated using the Landsat 7 ETM+ and Landsat 8 OLI panchromatic band images and Sentinel-1 intensity images. The glacier terminus was detected manually based on the different reflectances of the ice front, sea ice, and seawater. After mapping the terminus, temporal variations in the terminus position were calculated using the common box method [38]. This method used an open-ended box that approximately delineated the sides of the glacier to calculate the glacier area in this box, with a chosen reference line and the terminus as its upper and lower ranges. Temporal variations in the terminus position were estimated by dividing the calculated glacier area by the width of the box. Errors in the terminus positions were mainly caused by inaccuracy in the mapping of the glacier terminus. All Landsat images were accurately coregistered with the Landsat 8 image in 2017 during preprocessing. The errors in the terminus positions were controlled within one pixel (<15 m).

4.2. Surface Velocity

4.2.1. Velocity from Landsat We tracked features in pairs of Landsat images acquired in austral summer at approximately one-year intervals during 2000–2019 to calculate the annual horizontal displacement of Dalk Glacier using the COSI-Corr (Co-registration of Optically Sensed Images and Correlation) [39,40]. The scan gaps, which account for approximately 22% of SLC-Off Landsat 7 images, were filled using a local linear histogram matching technique [41]. High-precision coregistration of Landsat 7 images was performed manually using the Landsat 8 image acquired in 2013 as the reference. The well-distributed tie points (30–50) were carefully selected and optimized from stationary features, such as bedrock outcrops or 1 slow-moving (0–10 m a− ) areas. Once the above preprocessing was completed, cross-correlation analysis was performed by a Fourier-based frequency correlator to extract displacements along the East/West (X) and North/South (Y) directions. The cross-correlation process was iteratively conducted from a large search window to a smaller window. To ensure the efficiency of this operation and the accuracy of matching, we determined the optimal window sizes after extensive tests. The sizes of the initial search window (tracking patch) and the final window were finally set as 256 256 and × 64 64 pixels, respectively. The tracking step was set as 8 pixels in both the X and Y directions, which × means that the resolution of the velocity map is 120 120 m. × 4.2.2. Velocity from Sentinel-1 To retrieve monthly surface velocities over Dalk Glacier, we used the GAMMA software [42,43] to apply the intensity offset tracking algorithm [44] with 20 scenes of Sentinel-1 synthetic aperture radar (SAR) images acquired from August 2016 to September 2017. The fine coregistration and deramping procedures are required before offset tracking [42,45]. We adopted a coregistration strategy that considers the topography (REMA). After obtaining the DEM in SAR coordinates, we derived the coregistration lookup table, which was then refined by correcting the offset between the transformed master and slave images. The optimized lookup table and offset polynomials estimated by matching algorithm were used to coregister the image pairs, and the residual offsets were confined within 0.02 pixels. Subsequently, we deramped each TOPS-mode data for the azimuth spectrum ramp before applying oversampling. In the offset tracking process, different search windows were tested, and the window size of 640 128 pixels was finally selected. The tracking step was set as 100 20 pixels in × × the range and azimuth directions. We used REMA to geocode velocity maps with a bicubic-log spline interpolation and regularize the velocity field into a 200 m grid. Remote Sens. 2020, 12, 1809 5 of 18

4.2.3. Filtering and Calibration Erroneous matches are common in raw velocity fields obtained from feature tracking or offset tracking. We used a statistical filtering methodology to remove mismatches [46]. First, we computed the median and interquartile range (IQR) from the raw two-direction displacement fields in patches (5 5 pixels). Speed greater than 0.5-fold IQR from the median was discarded in each patch. × The mean displacement field (dx and dy) and the gradient threshold (σdx and σdy) were estimated from the remaining values. Finally, raw velocity fields with σdx and σdy larger than 30 m were discarded. Despite the high geolocation accuracy of Landsat images, residual geolocation errors (3–10 m for Landsat 8) introduce some offsets into the tracking algorithm [47]. We used the method reported by Fahnestock et al. [47] to correct the potential biases in the velocity maps caused by geolocation errors. This method takes advantage of the fact that most geolocation errors between multi-temporal Landsat 8 images are presented as plane shifts. We calculated the average X and Y displacement on the ice-free stable ground (exposed bedrocks). The X and Y shifts were then performed to calibrate each velocity field. Given that we coregistered the Landsat 7 images to Landsat 8, we simply calibrated all Landsat velocities using the same method.

4.2.4. Error Analysis and Time Series Extraction The errors of final surface velocity are strongly dependent on coregistration accuracy and time intervals between images in a pair [47,48]. We analyzed velocity errors by testing image pairs with different time intervals and from varying sensors. Similar to Wendleder et al. [49], the error and corresponding standard deviation ( 1σ) of each surface ice velocity were calculated by extracting ± residual speed at widely distributed non-moving points over the exposed bedrocks. Given the different spatial coverage of sensors, the number of control points varied from 100 for Landsat to 200 for Sentinel-1. Table2 shows the mean velocity errors for each sensor and time interval.

Table 2. Errors analysis of surface velocities averaged for each sensor and time interval.

Landsat 7 Landsat 8 Sentinel-1 Interval (days) 352/368 352/368 12/24 1 Error (m a− ) 5.63/3.92 2.24/1.73 7.62/4.03 Standard deviation( 1σ) 7.63/6.45 4.54/3.48 8.12/5.87 ±

We used three rectangular boxes (approximately 1 km 1 km) to extract and analyze the ice × velocity time series of different regions at Dalk Glacier. The front region (FT), grounding line region (GL), and upstream region (UP) are located between 8 and 9 km downstream, 0.5 and 0.5 km, − and 2 and 3 km upstream from the grounding line, respectively. The FT region is located behind the southernmost terminus position. We used this region selection strategy to obtain effective velocity values and improve the credibility of the variation analysis.

4.3. Surface Elevation and Ice Thickness The Worldview DEM (WVDEM) was generated using the LPS (IMAGINE Photogrammetry) tool in ERDAS Imagine. The ancillary data provided with the images were imported to obtain the initial orientation of each image. Tie points were generated automatically with a search size of 81 pixels in most areas and added manually in large slope regions. The final DEM with 5 m resolution was extracted and improved after iterative point collection and rational function refinement. The calculated WVDEMs were geodetic heights, which were relative to the WGS84 ellipsoid. We used the ICESat data acquired in 2008 and the REMA dataset to validate the WVDEM. In comparison with REMA, we analyzed the differences in the range of the entire scene and the ice tongue. All mean differences and standard deviations are shown in Table3. Remote Sens. 2020, 12, 1809 6 of 18

Table 3. Accuracy analysis of the Worldview DEM.

Comparison Dataset Mean Difference (m) Standard Deviation (m) ICESat 0.54 3.60 REMA (whole scene) 1.23 5.94 − REMA(ice tongue) 0.47 1.60

To assess the floating conditions, the ice thickness of Dalk Glacier was estimated using the WVDEM. On the basis of the principle of hydrostatic equilibrium (HE), the ice thickness of the free-floating ice tongue at Dalk Glacier can be determined from its freeboard height [50]: " #   ρw Z = h f b h f n + h f n (1) − ρw ρ − i where Z is the ice thickness, hfb is the freeboard height, hfn is the firn density correction, ρw is the density 3 of seawater, and ρi is the ice density. The densities of seawater and ice were set as 1027 and 917 kg m− , respectively [51]. The firn density correction was adopted from a firn densification model (FDM) and resampled to 1 km grid spacing [52,53]. The freeboard height was calculated by subtracting the local sea surface geodetic height, which was the mean height in the bay measured by ICESat during 2004–2008, from the WVDEM.

5. Results

5.1. Terminus Positions and Calving Events Figure2 indicates that Dalk Glacier presents an alternating pattern between advance and retreat from 2003 to 2017. The glacier terminus retreated in 2006, 2009, 2010, and 2016 and advanced in other years. Taking the first year of continuous advance as the dividing year, we categorized terminus position changes from 2003 to 2017 into three stages. The periods from 2003 to 2007 and from 2011 to 2017 belonged to the same type, in which large areas of glacial ice disintegrated after 3–5 years of terminus advance. The readvance was interrupted by sporadic calving events in the stage from 2007 to 2011. We monitored the minimum glacier extent in 2011 (31.24 km2) and the maximum glacier extent (33.55 km2) in 2016. The most significant retreat occurred in 2016, with an area reduction of approximately 2.19 km2. Massive icebergs calved in February and March 2016, and subsequent fragmentation during the austral winter led to the largest area loss in this period. Many calving events were observed from 2000 to 2019. Multisource satellite images between 11 January 2016 and 7 September 2016 illustrated the evolution of calving events in 2016 (AppendixA Figure A1). In the initial stage, an east section (~1.1 km2) of ice had broken away from the ice front before 28 February. An approximately 0.6 km2 section of the western ice tongue disintegrated before 31 March 2016. Throughout the austral winter of 2016, the terminus of Dalk Glacier had no evident calving until 31 August. For 19–31 August 2016, significant fracturing in the glacier terminus occurred, which caused a considerable amount of ice to break up, and a large amount of ice mélange formed in the glacier . The preferential disintegration on the east side of the ice front may be related to the near-shore current from east to west. Remote Sens. 2020, 12, x FOR PEER REVIEW 6 of 17

sea surface geodetic height, which was the mean height in the bay measured by ICESat during 2004– 2008, from the WVDEM.

5. Results

5.1. Terminus Positions and Calving Events Figure 2 indicates that Dalk Glacier presents an alternating pattern between advance and retreat from 2003 to 2017. The glacier terminus retreated in 2006, 2009, 2010, and 2016 and advanced in other years. Taking the first year of continuous advance as the dividing year, we categorized terminus position changes from 2003 to 2017 into three stages. The periods from 2003 to 2007 and from 2011 to 2017 belonged to the same type, in which large areas of glacial ice disintegrated after 3–5 years of terminus advance. The readvance was interrupted by sporadic calving events in the stage from 2007 to 2011. We monitored the minimum glacier extent in 2011 (31.24 km2) and the maximum glacier extent (33.55 km2) in 2016. The most significant retreat occurred in 2016, with an area reduction of Remoteapproximately Sens. 2020, 12 2.19, 1809 km2. Massive icebergs calved in February and March 2016, and subsequent7 of 18 fragmentation during the austral winter led to the largest area loss in this period.

Figure 2. Three advance and retreat stages of Dalk Glacier during 2003–2017. The background is theFigure Worldview-2 2. Three advance image acquired and retreat on 2stages January of Dalk 2016. Gl Theacier black during arrows 2003-2017. represent The the background glacier advance, is the whileWorldview-2 the red arrowsimage representacquired theon 2 terminus January retreat. 2016. The black arrows represent the glacier advance, while the red arrows represent the terminus retreat. 5.2. Surface Velocity FromMany Landsat calving 7events/8 and were Sentinel-1 observed images, from we 2000 obtained to 2019. 15 Multisource annual ice velocity satellite results images between between 2000 11 andJanuary 2019 2016 and and 13 monthly 7 September ice velocity 2016 illustrated results from the evol Augustution 2016 of calving to September events in 2017. 2016 Given(Figure the A1). lack In ofthe suitable initial stage, images, an the east annual section ice (~1.1 velocities km2) forof 2001,ice had 2002, broken 2003, away 2010, from and 2013the ice are front missing before in our 28 study.February. We findAn approximately that long interval 0.6 pairskm2 section in the tracking of the western algorithm ice leadtongue to smalldisintegrated errors in before the final 31 surface March velocity (Table2). The mean errors of the Sentinel-1 velocity derived from image pairs with 12 and

1 24-day intervals are 7.62 and 4.03 m a− , respectively. The errors of the Landsat 8 velocity with intervals 1 of 352 and 368 days are 2.24 and 1.73 m a− , respectively, while the Landsat 7 velocity errors with 1 the same time intervals are 5.63 and 3.92 m a− , respectively. The accuracy of ice velocity calculated from Landsat 8 is higher than that extracted from Landsat 7. The magnitude of these velocity errors is comparable with those reported in the scientific literatures [49,54]. The surface velocity maps show the flow characteristics of Dalk Glacier (Figure3): the 1 narrows as it crosses the grounding line, forming a floating ice tongue with a mean speed of ~170 m a− 1 and a maximum speed at the terminus of ~220 m a− . Another relatively fast ice flow in the upstream region appears approximately 1 km away from the grounding line. We failed to estimate the ice velocity in the west side and southeastern convergence regions of Dalk Glacier, which may be caused by the turning of the ice stream and steep terrain. Remote Sens. 2020, 12, x FOR PEER REVIEW 7 of 17

2016. Throughout the austral winter of 2016, the terminus of Dalk Glacier had no evident calving until 31 August. For 19–31 August 2016, significant fracturing in the glacier terminus occurred, which caused a considerable amount of ice to break up, and a large amount of ice mélange formed in the glacier fjord. The preferential disintegration on the east side of the ice front may be related to the near-shore current from east to west.

5.2. Surface Velocity From Landsat 7/8 and Sentinel-1 images, we obtained 15 annual ice velocity results between 2000 and 2019 and 13 monthly ice velocity results from August 2016 to September 2017. Given the lack of suitable images, the annual ice velocities for 2001, 2002, 2003, 2010, and 2013 are missing in our study. We find that long interval pairs in the tracking algorithm lead to small errors in the final surface velocity (Table 2). The mean errors of the Sentinel-1 velocity derived from image pairs with 12 and 24-day intervals are 7.62 and 4.03 m a− 1, respectively. The errors of the Landsat 8 velocity with intervals of 352 and 368 days are 2.24 and 1.73 m a− 1, respectively, while the Landsat 7 velocity errors with the same time intervals are 5.63 and 3.92 m a− 1, respectively. The accuracy of ice velocity calculated from Landsat 8 is higher than that extracted from Landsat 7. The magnitude of these velocity errors is comparable with those reported in the scientific literatures [49,54]. The surface velocity maps show the flow characteristics of Dalk Glacier (Figure 3): the ice stream narrows as it crosses the grounding line, forming a floating ice tongue with a mean speed of ~170 m a− 1 and a maximum speed at the terminus of ~220 m a− 1. Another relatively fast ice flow in the upstream region appears approximately 1 km away from the grounding line. We failed to estimate Remote Sens. 2020, 12, 1809 8 of 18 the ice velocity in the west side and southeastern convergence regions of Dalk Glacier, which may be caused by the turning of the ice stream and steep terrain.

Figure 3. Surface ice velocity at Dalk Glacier. (a) Surface velocity map derived from Landsat 7 images Figure 3. Surface ice velocity at Dalk Glacier. (a) Surface velocity map derived from Landsat 7 images (20050221/20060224). The black and pink lines are the grounding line and southernmost terminus (20050221/20060224). The black and pink lines are the grounding line and southernmost terminus position on 28 January 2011. The black dashed rectangular boxes are regions for temporal velocity position on 28 January 2011. The black dashed rectangular boxes are regions for temporal velocity analysis. (b) Surface ice velocity derived from Sentinel-1 images (20160807/20160831). AA’ is the location analysis. (b) Surface ice velocity derived from Sentinel-1 images (20160807/20160831). AA’ is the of the centerline profile. (c) and (d) are interannual velocity variations along AA’ before and after large location of the centerline profile. (c) and (d) are interannual velocity variations along AA’ before and calvings in 2006 and 2016, respectively. The black dashed line shows the position of the grounding line.

The annual velocity profiles along the centerline (AA’) that were calculated for 2005–2009 and 2015–2019 reveal the interannual velocity variation before and after large calving events in 2006 and 1 2016 (Figure3). The largest velocities of Dalk Glacier reached ~230 m a − at the terminus in 2007 and 2017. We found that the surface velocity in 2005–2009 and 2015–2019 showed similar interannual change patterns, with a speedup in the years (2007 and 2017) following large calvings (2006 and 2016). It slowed down to the pre-calving speed in the subsequent years. The velocity fluctuation of the glacier front was significant, whereas the change in the grounding area was small. To further examine the interannual and seasonal velocity changes, we analyzed the time series of the averaged speeds of three chosen regions (Figure4). For the years 2000 to 2019, we observed interannual changes in all three regions of Dalk Glacier. Significant velocity increases of approximately 25 3.9 m a 1 (~12%) in the FT region in 2007, 2012, and 2017 were observed, while the velocities of ± − the GL and UP regions increased by approximately 10 8.7 m a 1 (~6%) in 2012. In 2017, the velocity ± − of the GL and UP regions increased slightly. Remote Sens. 2020, 12, x FOR PEER REVIEW 8 of 17

after large calvings in 2006 and 2016, respectively. The black dashed line shows the position of the grounding line.

The annual velocity profiles along the centerline (AA’) that were calculated for 2005–2009 and 2015–2019 reveal the interannual velocity variation before and after large calving events in 2006 and 2016 (Figure 3). The largest velocities of Dalk Glacier reached ~230 m a− 1 at the terminus in 2007 and 2017. We found that the surface velocity in 2005–2009 and 2015–2019 showed similar interannual change patterns, with a speedup in the years (2007 and 2017) following large calvings (2006 and 2016). It slowed down to the pre-calving speed in the subsequent years. The velocity fluctuation of the glacier front was significant, whereas the change in the grounding area was small. To further examine the interannual and seasonal velocity changes, we analyzed the time series of the averaged speeds of three chosen regions (Figure 4). For the years 2000 to 2019, we observed interannual changes in all three regions of Dalk Glacier. Significant velocity increases of − 1 approximatelyRemote Sens. 2020, 1225, 1809± 3.9 m a (~12%) in the FT region in 2007, 2012, and 2017 were observed, while9 of 18

FigureFigure 4. VariationsVariations in the ice velocity of Dalk Glacier. (a) (a) Interannual Interannual velocity velocity changes changes at at the the front front (FT),(FT), grounding grounding line line (GL), and upstream (UP) regionsregions in 2000–2019. (b) (b) Seasonal velocity variations fromfrom October October 2015 2015 to to September September 2017. 2017. Vertical Vertical bars bars are are error error bars bars and and horizontal horizontal bars bars are are temporal baselinesbaselines of the image pairs used in velocity calculation.

OnOn thethe basisbasis of of the the GoLIVE GoLIVE dataset dataset from from October Octo 2015ber 2015 to February to February 2017 and 2017 the and Sentinel-1 the Sentinel-1 monthly monthlyvelocities velocities from August from 2016August to September2016 to September 2017 (Figure 2017 (Figure4b), we 4b), found we thatfound the that ice the velocity ice velocity at Dalk at DalkGlacier Glacier had ahad significant a significant increase increase in the in austral the austral summer. summer. The The GoLIVE GoLIVE dataset dataset matched matched well well with with our ourSentinel-1 Sentinel-1 velocity velocity at monthlyat monthly scale. scale. From From the the austral austral spring spring in in 2015 2015 to to winterwinter 2016,2016, the FT region beganbegan to to accelerate accelerate in in November November 2015 2015 and and reache reachedd its itsmaximum maximum in February in February 2016. 2016. From From August August 2016 to2016 September to September 2017, 2017,the ice the velocity ice velocity increased increased from October/November from October/November 2016, peaked 2016, peaked in March in March 2017, and2017, decreased and decreased rapidly rapidly from March from Marchto May to 2017. May In 2017. addition, In addition, the summer the summer acceleration acceleration of Dalk Glacier of Dalk increasedGlacier increased after calving after calvingevents. events.In 2015/2016, In 2015 the/2016, seasonal the seasonal velocity velocity fluctuations fluctuations of the of FT the and FT andGL 1 GL regions were ~30 (~16%) and ~10 m a− (~6%), respectively. The Sentinel-1 monthly velocities showed a seasonal acceleration of ~35 6.6 (~19%) and ~15 6.6 m a 1 (~9%) in the FT and GL regions, ± ± − respectively, in 2016/2017.

5.3. Elevations and Ice Rumple Figure5a illustrates the surface elevation of Dalk Glacier (WVDEM) derived from the Worldview in-track stereo image pair. The glacier surface elevation ranges from 50 m to 550 m. The ice tongue is relatively flat at approximately 100–130 m. The topography of Dalk Glacier differs significantly between the east and west. The east side of the glacier is steeper than the west side, and the west side of the glacier has many exposed rocks near the coastline. The agreement between the WVDEM and REMA was highest in low-slope areas (Figure5b). Abnormal elevation pixels with approximately 20 m errors appeared in steep regions, mostly in the eastern part of Dalk Glacier. In comparison with REMA, the mean difference and standard deviation were 1.23 and 5.94 m (Table3), respectively, over − the whole scene, but 0.47 and 1.6 m, respectively, at the ice tongue. Remote Sens. 2020, 12, x FOR PEER REVIEW 9 of 17

regions were ~30 (~16%) and ~10 m a− 1 (~6%), respectively. The Sentinel-1 monthly velocities showed a seasonal acceleration of ~35 ± 6.6 (~19%) and ~15 ± 6.6 m a− 1 (~9%) in the FT and GL regions, respectively, in 2016/2017.

5.3. Elevations and Ice Rumple Figure 5a illustrates the surface elevation of Dalk Glacier (WVDEM) derived from the Worldview in-track stereo image pair. The glacier surface elevation ranges from 50 m to 550 m. The ice tongue is relatively flat at approximately 100–130 m. The topography of Dalk Glacier differs significantly between the east and west. The east side of the glacier is steeper than the west side, and the west side of the glacier has many exposed rocks near the coastline. The agreement between the WVDEM and REMA was highest in low-slope areas (Figure 5b). Abnormal elevation pixels with approximately 20 m errors appeared in steep regions, mostly in the eastern part of Dalk Glacier. In Remote Sens. 2020, 12, 1809 10 of 18 comparison with REMA, the mean difference and standard deviation were −1.23 and 5.94 m (Table 3), respectively, over the whole scene, but 0.47 and 1.6 m, respectively, at the ice tongue.

Figure 5. a Figure 5.( (a)) Surface Surface elevation elevation of of Dalk Dalk Glacier.Glacier. Black line liness are are contours contours with with 50-m 50-m intervals. intervals. Black Black dots dots are footprints of ICESat data. The pink line donates the possible ice rumple border. (b) Elevation are footprints of ICESat data. The pink line donates the possible ice rumple border. (b) Elevation difference between WVDEM and the Reference Elevation Model of Antarctica (REMA). The grey line difference between WVDEM and the Reference Elevation Model of Antarctica (REMA). The grey line shows the position of the grounding line. shows the position of the grounding line. From the WVDEM, we detected a small “ice bump” (~2 km 1 km) at the front portion of Dalk From the WVDEM, we detected a small “ice bump” (~2 km× × 1 km) at the front portion of Dalk Glacier. The enlarged view of this special landscape is shown in Figure6. Along the longitudinal Glacier. The enlarged view of this special landscape is shown in Figure 6. Along the longitudinal profile (BB’), the upstream surface elevation decreased from 110 m to 65 m and then increased by profile (BB’), the upstream surface elevation decreased from 110 m to 65 m and then increased by approximately 30 m. The elevation of the inflection point where the surface terrain changed from approximately 30 m. The elevation of the inflection point where the surface terrain changed from descent to ascent was ~65 m. Thus, we used the 65 m contour as a threshold to extract the possible descent to ascent was ~65 m. Thus, we used the 65 m contour as a threshold to extract the possible range of the “ice bump”. From the surface elevation and flow characteristics, we speculate that the “ice range of the “ice bump”. From the surface elevation and flow characteristics, we speculate that the bump” is an ice rumple. First, it is very small and rises only a few tens of meters. Second, the ice Remote“ice Sens. bump” 2020, 12is , anx FOR ice PEERrumple. REVIEW First, it is very small and rises only a few tens of meters. Second,10 ofthe 17 ice tongue flows across it without stagnation. These features are typical of ice rumples in Antarctica [24]. tongue flows across it without stagnation. These features are typical of ice rumples in Antarctica [24].

Figure 6. Overview of the ice rumple at the terminus of Dalk Glacier. (a) DEM of the glacier terminus Figureat Dalk 6. Glacier. Overview (b) of Elevation the ice rumple along theat the longitudinal terminus of profile Dalk Glacier. (BB’). The (a) pinkDEM lineof the shows glacier the terminus possible atrange Dalk of Glacier. the ice rumple.(b) Elevation along the longitudinal profile (BB’). The pink line shows the possible range of the ice rumple. To confirm the ice rumple further, we analyzed the floating status and potential pinning points of theTo ice confirm front. the When ice rumple the bottom further, of thewe iceanalyzed tongue the comes floating into status contact and with potential the pinning pinning point points or of the ice front. When the bottom of the ice tongue comes into contact with the pinning point or bedrock, the ice rumple forms on the upper surface [27]. We evaluated the floating status by comparing the equivalent ice tongue thickness calculated based on the HE assumption and the historical airborne radio-echo sounding (RES) measurements of the actual ice thickness (Figure 7). The HE assumption is valid if the ice tongue is freely floating. Therefore, the anomalous ice tongue area with a large difference between the equivalent ice thickness and the actual ice thickness can be regarded as the grounded region. Given that the error of the WVDEM at the ice tongue is < 4 m and ±1–3 m FDM error in coastal areas [53], the accuracy of the WVDEM-derived ice thickness is better than 44.76 m based on error propagation. Considering the uncertainty of RES (~75 m) [37] and statistics of the differences between equivalent ice thickness and RES data in AIS (mean error minus three times the standard deviation is −209.45 m) [55], we used the large threshold of ±209.45 m to estimate the ice thickness anomaly area. As illustrated in Figure 7, the HE assumption breaks down at the terminus, which demonstrates possible frontal pinning points and a grounding ice rumple.

Figure 7. (a) Ice thicknesses of Dalk Glacier derived from WVDEM. Triangle points denote the locations of radio-echo sounding (RES) data. Points in black ellipsoid indicate floating regions, while others reveal grounded regions. (b) Comparison between RES and WVDEM-derived ice thickness. Points in black ellipsoid correspond to points marked with ellipsoid in (a).

Remote Sens. 2020, 12, x FOR PEER REVIEW 10 of 17

Figure 6. Overview of the ice rumple at the terminus of Dalk Glacier. (a) DEM of the glacier terminus at Dalk Glacier. (b) Elevation along the longitudinal profile (BB’). The pink line shows the possible range of the ice rumple. Remote Sens. 2020, 12, 1809 11 of 18 To confirm the ice rumple further, we analyzed the floating status and potential pinning points of the ice front. When the bottom of the ice tongue comes into contact with the pinning point or bedrock,bedrock, the icethe rumple ice rumple forms forms on the upperon the surface upper [ 27surface]. We evaluated[27]. We theevaluated floating statusthe floating by comparing status by thecomparing equivalent icethe tongue equivalent thickness ice tongue calculated thickness based onca thelculated HE assumption based on the and HE the historicalassumption airborne and the radio-echohistorical sounding airborne (RES) radio-echo measurements sounding of (RES) the actual meas iceurements thickness of (Figure the actual7). The ice HEthickness assumption (Figure is 7). validThe if theHE iceassumption tongue is freelyis valid floating. if the ice Therefore, tongue theis fr anomalouseely floating. ice Therefore, tongue area the with anomalous a large di fficeerence tongue betweenarea with the equivalenta large difference ice thickness between and the the equivalent actual ice ice thickness thickness can and be the regarded actual ice as thethickness grounded can be region. Given that the error of the WVDEM at the ice tongue is < 4 m and 1–3 m FDM error in coastal regarded as the grounded region. Given that the error of the WVDEM± at the ice tongue is < 4 m and areas±1–3 [53 m], theFDM accuracy error in ofcoastal the WVDEM-derived areas [53], the accuracy ice thickness of the isWVDEM-derived better than 44.76 ice m thickness based on is error better propagation.than 44.76 Considering m based on the error uncertainty propagation. of RES Considering (~75 m) [37] the and uncertainty statistics of theof diRESfferences (~75 m) between [37] and equivalentstatistics ice of thicknessthe differences and RES between data inequivalent AIS (mean ice error thickness minus and three RES times data thein AIS standard (mean deviation error minus is 209.45 m) [55], we used the large threshold of 209.45 m to estimate the ice thickness anomaly −three times the standard deviation is −209.45 m)± [55], we used the large threshold of ±209.45 m to area.estimate As illustrated the ice thickness in Figure7 anomaly, the HE assumptionarea. As illustra breaksted down in Figure at the 7, terminus,the HE assumption which demonstrates breaks down possibleat the frontalterminus, pinning which points demonstrates and a grounding possible icefrontal rumple. pinning points and a grounding ice rumple.

Figure 7. (a) Ice thicknesses of Dalk Glacier derived from WVDEM. Triangle points denote the locations of radio-echoFigure 7. (a) sounding Ice thicknesses (RES) data. of Dalk Points Glacier in black derived ellipsoid from indicate WVDEM. floating Triangle regions, points while denote others the reveallocations grounded of radio-echo regions. ( bsounding) Comparison (RES) between data. Points RES in and black WVDEM-derived ellipsoid indicate ice thickness.floating regions, Points while in blackothers ellipsoid reveal correspond grounded to regions. points marked(b) Comparison with ellipsoid between in (a). RES and WVDEM-derived ice thickness. Points in black ellipsoid correspond to points marked with ellipsoid in (a). 5.4. Linkage between Terminus Change, Velocity Variation, and Ice Rumple Our analyses indicate that the acceleration of Dalk Glacier is correlated with the terminus retreat (Figure8a). During the retreat stage, Dalk Glacier usually underwent major disintegrations within

1 year (2006 and 2016) or sporadic disintegrations that can last for 2 years (2009–2010). In 2007, 2011, and 2017, the increase in the speed of the FT region occurred when the terminus retreated to a similar position (6.7–6.9 km). The accelerations of GL and UP areas in 2012 and 2017 were also likely to be related to calving events. In the advancing stage of the glacier, the FT ice velocity usually decreased to pre-calving rates and then remained relatively stable. The surface velocity at the glacier front decreased by approximately 20 4.3 m a 1 in 2008, 2014, and 2018, while no significant change was observed in ± − speeds from 2009 to 2011, 2014 to 2016, and 2018 to 2019. Therefore, the interannual velocity variations and terminus changes of Dalk Glacier presented similar periodicity (approximately 5 years). Remote Sens. 2020, 12, x FOR PEER REVIEW 11 of 17

5.4. Linkage between Terminus Change, Velocity Variation, and Ice Rumple Our analyses indicate that the acceleration of Dalk Glacier is correlated with the terminus retreat (Figure 8a). During the retreat stage, Dalk Glacier usually underwent major disintegrations within 1 year (2006 and 2016) or sporadic disintegrations that can last for 2 years (2009–2010). In 2007, 2011, and 2017, the increase in the speed of the FT region occurred when the terminus retreated to a similar position (6.7–6.9 km). The accelerations of GL and UP areas in 2012 and 2017 were also likely to be related to calving events. In the advancing stage of the glacier, the FT ice velocity usually decreased to pre-calving rates and then remained relatively stable. The surface velocity at the glacier front decreased by approximately 20 ± 4.3 m a− 1 in 2008, 2014, and 2018, while no significant change was observed in speeds from 2009 to 2011, 2014 to 2016, and 2018 to 2019. Therefore, the interannual Remote Sens. 2020, 12, 1809 12 of 18 velocity variations and terminus changes of Dalk Glacier presented similar periodicity (approximately 5 years).

Figure 8. Link between terminus change, velocity variation, and ice rumple. (a) Terminus changes and velocity variations of Dalk Glacier in 2004–2017. Short grey lines donate the overlay time of image Figure 8. Link between terminus change, velocity variation, and ice rumple. (a) Terminus changes pairs used in velocity calculation. (b) Positions of glacier terminus and the ice rumple. The pink dotted and velocity variations of Dalk Glacier in 2004–2017. Short grey lines donate the overlay time of image line shows the possible range of the ice rumple. Colored solid lines are the maximum and minimum pairs used in velocity calculation. (b) Positions of glacier terminus and the ice rumple. The pink dotted glacier extents for each advance and retreat phase. line shows the possible range of the ice rumple. Colored solid lines are the maximum and minimum Ourglacier observations extents for each also advance demonstrated and retreat the phase. possible link between the ice rumple and the terminus changes (Figure8b). The ice rumple blocked the ice flow, forming transverse and weakening the stabilityOur observations of the glacier also terminus. demonstrated Consequently, the possibl periodice link between calving the events ice rumple occurred. and By the analyzing terminus thechanges maximum (Figure glacier 8b). extent The ice in eachrumple cycle, blocked we found the thatice retreatflow, forming or calving transverse usually occurred crevasses when and theweakening terminus the advanced stability to of the the seaward glacier terminus. edge of the Consequently, ice rumple. Theperiodic minimum calving glacier events extent occurred. in each By cycleanalyzing shows the that maximum the terminus glacier eventually extent in retreatseach cycle, to the we landward found that edge retreat of theor calving ice rumple. usually occurred when the terminus advanced to the seaward edge of the ice rumple. The minimum glacier extent in 6.each Discussion cycle shows that the terminus eventually retreats to the landward edge of the ice rumple. The Sentinel-1 mission has allowed nearly continuous monitoring of Dalk Glacier since 2016, o ffering the capability of detecting calving events in polar night and seasonal or sub-seasonal velocity fluctuations. The combination of optical data and SAR data not only overcomes the effects of cloud and night but also improves the details of change detection. Stereo images from Worldview-1 are particularly successful in obtaining high-precision and high-resolution DEM without control points, which can be utilized to investigate the complete ice tongue elevation at a specific moment and the first observation of the ice rumple range at the front of Dalk Glacier. Our results are in good agreement with the previous assessment of changes in the terminus and ice flow of Dalk Glacier [31,33]. Liu et al. [31] analyzed the terminus positions and observed the pattern of alternating between advance and retreat. Our study confirmed their observations and added more details to the terminus positions between 2000 and 2019. Relatively large disintegrations occurred Remote Sens. 2020, 12, 1809 13 of 18 in 2006 and 2016, resulting in the significant retreats of the terminus, while sporadic disintegrations were observed from 2009 to 2011. We found a 3–5 year calving cycle of the ice tongue. Besides, we observed that the terminus also disintegrated many times between austral summer and winter in 2016. Ai et al. [33] found seasonal velocity changes at the glacier front from 2007 to 2012. This conclusion agreed with our observed seasonal velocity fluctuations. In the summers from 2015 to 2017, the ice flow of the ice tongue accelerated. Previous surface velocity studies focused only on short time series and single events. However, our studies, based on multisource remote sensing data, investigated the continuous, long-term interannual velocity time series and analyzed the detailed sub-seasonal velocity changes before and after the calving events. Our results show that the interannual glacier speedup underlies a 5-year cycle. In 2007, 2012, and 2017, the annual velocity of the glacier front increased. After the disintegrations in 2016, we also detected that the seasonal speed fluctuations increased by ~4.5%. The surface velocity of Dalk Glacier showed significant seasonal variations. According to other studies of ice shelves and glaciers in East Antarctica, the increased velocities in the austral summer are very likely related to sea ice conditions. The seasonal variability of the Totten has been confirmed to be closely linked to the presence of sea ice at the ice shelf front [56]. Liang et al. [21] also reported that the seasonal velocity variations of the Polar Record Glacier (PRG) are affected by the change of sea ice. PRG and Dalk Glacier are just ~50 kilometers apart, and they are both located in Prydz Bay. Considering the similar sea ice conditions at the front of neighboring glaciers, we suppose that the seasonal velocity change of Dalk Glacier is also affected by the sea ice change. The analysis of interannual velocity variations and terminus position changes indicates that the speedup of Dalk Glacier is correlated with the terminus retreat. The accelerations in 2007 and 2017 were linked to major disintegrations in 2006 and 2016, whereas the surface velocity increased in 2012 after continuous small-scale calvings in 2009 and 2010. The commonality of these acceleration phenomena is that the terminus undergoes a large magnitude of retreat and detaches from the observed ice rumple. We suspect that the acceleration after the terminus retreated to the landward edge of the ice rumple is resulted from the reduced buttressing of the ice rumple. Few studies have reported the impact of ice rumples on glacier flow, which may considerably buttress ice tongues and ice shelves [24], and retreat from regions of similar resistance elsewhere have led to accelerated ice flow [56]. Thus, we attribute the interannual acceleration of Dalk Glacier to the loss of buttressing from the separation of terminus and the ice rumple. Observations in this study demonstrate that the ice rumple plays an important role in the frequent ice-dynamical changes of Dalk Glacier. Under the impact of the ice rumple, interannual ice-flow accelerations and terminus position changes in Dalk Glacier have similar periodic patterns. The ice rumple influences the change in surface velocity. As mentioned above, the cycle acceleration is related to the periodic non-contact of this pinning point. The ice rumple also provides a significant constraint effect for the Dalk ice tongue. Callens et al. [26] reported that several ice rises and pinning points in West Ragnhild Glacier make its ice shelf locally grounded, which dominate the dynamics and ensure stability by providing buttressing and slowing down the flow upstream. In our cases, the ice rumple restrained and slowed down the Dalk ice tongue during the period of sustained terminus advance. As the ice tongue regained its advance and started contact with the ice rumple, the ice velocity gradually decreased to its pre-calving rates. In the years 2008–2011, 2013–2016, and 2018–2019, the increased terminus velocity decreased by about 20 4.3 m a 1 in the first year and remained relatively stable in ± − other years. The terminus position change is also linked to the ice rumple. Since the ice rumple is a locally elevated feature and located near the terminus, the stress generated in the locations where the ice contacts the rumple enhances ice fracture, as evident in the along-flow changes in crevassing in Figure6b. Although uncertainties exist in the method of extracting the ice rumple range using a single surface elevation, the impact of ice rumple on the terminus change is still evident. The southernmost terminus position in each cycle is near the landward edge of the ice rumple, while the northernmost terminus position is also limited. The 3–5 year calving cycle of Dalk Glacier is mainly controlled by Remote Sens. 2020, 12, 1809 14 of 18 the ice rumple. Calving is a complex process influenced by several factors, including atmospheric, oceanographic, and glaciological forcing [57]. The intrusion of warm modified circumpolar deep water (mCDW) into Prydz Bay [58,59] and a strong peak in ocean temperatures in 2010 at the neighboring PRG [21] were observed. These independent observations of ocean warming suggest that oceanic forcing was the possible cause of the sporadic disintegrations in 2009 and 2010.

7. Conclusions In this study, a comprehensive overview of dynamic changes in Dalk Glacier since 2000 was performed using multisource satellite data. The measurements of the glacier terminus position and surface velocity were conducted. In 2000–2019, the alternation between the advances and retreats of calving fronts, significant interannual accelerations, and increased seasonal fluctuations in surface velocity were monitored. Our results show the cyclic behavior of Dalk Glacier changes. The terminus position underlies a 3–5 year cycle, with the glacier retreating in 2006, 2009, 2010, and 2016 and advancing in other years. The surface velocity of Dalk Glacier underlies a 5-year cycle with interannual speed-ups in 2007, 2012, and 2017. Our study indicates that surface velocity changes at Dalk Glacier have a certain relationship with its terminus positions. The speedup often occurs after calving events and retreats. In addition, disintegrations contribute to the loss in buttressing of the ice rumple and cause corresponding increases in the velocities of different regions. Our study highlights the strong response of the dynamics of Dalk Glacier to locally grounding features. We found a kilometer-sized frontal ice rumple in Dalk Glacier that provides a significant buttressing effect and affects the terminus changes and surface velocity. The terminus of Dalk Glacier usually continues to retreat to the landward edge of the ice rumple region, followed by a significant increase in surface velocity. The restriction of the ice rumple causes the relatively stable speed in the glacial advance phases. Influenced by the ice rumple, the interannual velocity increases, and terminus variations have similar periodic patterns. We found that the calving of Dalk Glacier was influenced by the ice rumple. Normally, calving events occur when the ice tongue covers the complete ice rumple region, and the evolution is affected by coastal ocean current erosion.

Author Contributions: Y.C. and C.Z. jointly designed the study. Y.C. conducted the surface velocity and DEM calculations, wrote large parts of the manuscript, and compiled all figures. C.Z. helped to improve the analysis and the glaciological interpretation of the data. S.A. provides the interpretation of the ice rumple. Q.L. and L.Z. helped to improve the manuscript. R.L. mapped all glacier fronts. H.L. helped to analyze the ice thickness. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the National Natural Science Foundation of China (No. 41776200, 41531069 and 41941010), and the National Key Research and Development Program of China (2018YFC1406102). Acknowledgments: We acknowledge the European Space Agency for providing Sentinel-1 data. Landsat imagery was provided by the US Geological Survey Earth Resources Observation Science Center. The Worldview images were purchased from DigitalGlobe. The GLAS/ICESat L2 altimeter data was distributed by the National Snow and Ice Data Center Distributed Active Archive Center. Airborne radio-echo sounding (RES) measurements can be accessed using the Australian Antarctic Data Centre. The meteorological observation data sets were kindly provided by the Chinese National Arctic and Antarctica Data Center. Conflicts of Interest: The authors declare no conflict of interest. Remote Sens. 2020, 12, 1809 15 of 18

Remote Sens. 2020, 12, x FOR PEER REVIEW 14 of 17 Appendix A Appendix A

Figure A1. Landsat 8 and Sentinel-1 imagery showing the evolution of the largest calving event in Figure2016. A1. The Landsat red dashed 8 and line Sentinel-1 shows the imagery terminus showing position the on evolution 22 January of 2017.the largest calving event in 2016. The red dashed line shows the terminus position on 22 January 2017. References

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